Such pipe-lines are subject to thermically-induced changes in length (expansion or contraction) resulting from variations in the temperature of the pipe wall, particularly on their being put into and taken out of use. Totally rigid anchorage of such a pipe-line to parts of the structure may lead to the imposition of impermissibly high stresses on both the pipe-line itself and on the holding fixture. This applies particularly to the steam pipes in power stations. This is why, in order to obviate damage to pipe-lines and to avoid exceeding the permissible loading on junctions with components (e.g., boilers or turbines) connected to the pipe-lines, soft-mounted pipe guides are preferred.
This means that the pipe-line is held in flexible fixtures--i.e., fixtures capable of varying their length in a direction parallel to the pipe they are holding. Consequently the pipe-line is free to move, according to its thermic expansion or contraction, in relation to the structural member to which it is attached (expansion-conditioned displacement). Such holding fixtures can be designed as suspensory fixtures e.g., spring clamps or constant suspension mountings) or as supporting fixtures (e.g., spring rests).
In the course of their use, however, not all points of soft-mounted pipe-line systems move as the calculations might suggest. The reason for this is unpredictable frictional and relaxational effects in the pipe-line system.
These affect the kinetic behaviour of the pipe-lines in a manner that may lead to a considerable shortening of the service life of the pipe-line system. Such unpredictable influences on pipe-lines can be dealt with by means of thermically/mechanically acting "control suspension mountings" of the type familiar from DE-PS No. 27 45 473, the requisite vertical movements of the pipe being adjusted in conformity with actual temperature-conditioned changes in the length of the pipe. The way in which this is done is that the pipe-line is connected to the fulcrum of a bell-crank lever, the longer arm of which pivots on the building side point of suspension of a control rod running parallel to the axis of the pipe and separated from the influence of the temperature, while the shorter arm, lying at right angles to the axis of the pipe, is hinged to the control rod, the other end of which is connected to the pipe. While a change in the temperature of the pipe-line causes an alteration in the distance of the fulcrum of the bell-crank lever and the point of attachment of the other end of the control rod at the pipe-line, the length of the control rod itself and thus the distance between the control rod pivot point at the shorter arm and the point of attachment at the other end of the control rod remain unchanged because the control rod remains uninfluenced by the temperature of the pipe-line. For this reason the bell-crank lever is forced into a rotary movement about its apex, and corresponding to the situation of the lever this finally leads to a displacement of the fulcrum at right angles to the axis of the pipe (that is to say, to a change in the length of the control suspension mounting in the direction of suspension), since the position of the point of suspension at the end attached to the building (at the longer arm) remains unchanged.
Such control suspension mountings have proved to be very effective. They have the disadvantage, however, that they result in heavy and bulky constructions. In particular, it is often impossible to install them because the necessary length of the control rods (e.g. 3-4 meters) is too great for shorter lengths of pipe.
Moreover, the weight of the pipe-line induces considerable stresses in the pipe-line as a result of the leverage exerted through the control rods. It is true that, as taught in DE-PS No. 27 45 473, this effect is considerably alleviated by the installation of relief springs, but that in turn has the disadvantage that it increases the bulk of the construction still further.